Reference is hereby made to commonly assigned, co-pending U.S. patent application Ser. No. 09/999,890 filed on even date herewith for METHOD AND APPARATUS FOR DISCRIMINATING BETWEEN TACHYARRHYTHMIAS BY Bozidar Ferek-Petric.
This invention relates to cardiac implantable medical devices (IMDs) particularly adapted for developing a vectorcardiograph (VCG) from vector lead signals developed across selected pairs of implanted electrodes.
The mechanical events of the heart are preceded and initiated by the electrochemical activity of the heart (i.e., the propagation of the action potential). In the healthy heart, the electrical and mechanical operation of the heart is regulated by electrical signals produced by the heart""s sino-atrial (SA) node. Each signal produced by the SA node spreads across the atria, causing the depolarization and contraction of the atria, and arrives at the atrioventricular (A-V) node. The signal is then conducted to the xe2x80x9cBundle of Hisxe2x80x9d during which time it is slowed down to allow for the atrium to pump blood into the ventricles and thereafter to the xe2x80x9cBundle Branchesxe2x80x9d and the Purkinje muscle fibers of the right and left ventricles. The signals propagated through the Bundle Branches effects depolarization and accompanying contraction of the left ventricle and the right ventricle in close order. Following contraction, the myocardial cells repolarize during a short period of time, returning to their resting state. The right and left atria refill with venous and oxygenated blood, respectively, until the cardiac cycle is again commenced by a signal originating from the SA node. At rest, the normal adult SA node produces a signal approximately 60 to 85 times a minute, causing the heart muscle to contract, and thereby pumping blood to the remainder of the body. The electrical signal passes through the heart chambers as a wave front that can be characterized as a plane advancing from cell to cell through the cardiac muscle that separates cells of different electrical potential as a function of the time that it takes to complete the cardiac cycle.
The above-described cardiac cycle is disrupted in diseased or injured hearts, and the chronic or episodic disrupted electrical activity has long been employed to diagnose the state of the heart. A variety of techniques have been developed for collecting and interpreting data concerning the electrical activity of the heart both in the clinical setting and by way of portable external monitors carried by or IMDs implanted in an ambulatory patient to collect data relating to electrical heart function during daily activities of the patient. Such techniques include electrocardiography, vectorcardiography and polarcardiography.
The most commonly used of these techniques is the electrocardiograph (ECG) machine that displays one-dimension tracings of electrical signals of the heart as the depolarization wave front advances across the heart chambers in the cardiac cycle. An ECG machine typically measures and displays and/or records the voltages at various skin electrodes placed about the body relative to a designated xe2x80x9cgroundxe2x80x9d electrode. The paired electrodes are referred to as xe2x80x9cleadsxe2x80x9d and the lead signal is displayed or printed as an ECG lead tracing. The term xe2x80x9cleadxe2x80x9d would appear to indicate a physical wire, but in electrocardiography, xe2x80x9cleadxe2x80x9d actually means the electrical signal or vector in space between a designated pair of skin electrodes arranged as described below, wherein the vectors traverse the heart disposed between the skin electrodes.
The cardiac cycle as displayed in an ECG lead tracing reflects the electrical wave front as measured across one such ECG lead, as shown in U.S. Pat. No. 4,587,976, for example, and depicted in FIG. 1. The portion of a cardiac cycle representing atrial depolarization is referred to as a xe2x80x9cP-wave.xe2x80x9d Depolarization of the ventricular muscle fibers is represented by xe2x80x9cQxe2x80x9d, xe2x80x9cRxe2x80x9d, and xe2x80x9cSxe2x80x9d points of a cardiac cycle. Collectively these xe2x80x9cQRSxe2x80x9d points are called an xe2x80x9cR-wavexe2x80x9d or a xe2x80x9cQRS complex.xe2x80x9d Re-polarization of the depolarized heart cells occurs after the termination of another positive deflection following the QRS complex known as the xe2x80x9cT-wave.xe2x80x9d The QRS complex is the most studied part of the cardiac cycle and is considered to be the most important for the prediction of health and survivability of a patient. However, the time relation of the P-wave to the QRS complex and the height and polarity of the T-wave and S-T segment are also indicators of a healthy or diseased heart. The S-T segment of a healthy heart is usually isoelectric, i.e., neither positive nor negative in deflection from baseline of the ECG lead tracing. This S-T segment is a most important indicator of the health of the ventricular myocardium and is elevated in ischemia and due to infarctions disrupting the depolarization wave front.
The ECG machine typically plots each ECG lead in parallel over an interval of time such that the heart""s electrical activity for one or more cardiac cycles is displayed as parallel ECG lead tracings on a visual display screen and/or printed for purposes of monitoring or analysis. The most common ECGs are known as the xe2x80x9c12 leadxe2x80x9d, the xe2x80x9c18 lead,xe2x80x9d and a variety of other, fewer, lead combinations that simulate the more complete ECGs.
The 12-Lead system provides much redundant information in the frontal (X, Y) plane and transverse (X, Z) plane of the ECG vector signal. It permits only a rough visual estimate of the vector direction in theses two planes. Moreover, the number of skin electrodes and the bulk of the cables and the ECG machine make 12-lead and 18-lead ECG systems only practical in the clinical setting and impractical for use in a portable monitor for chronic use by a patient. Portable ECG recorders or xe2x80x9cHolter monitorsxe2x80x9d therefore employ fewer cables and electrodes to record at least certain of the above-listed ECG lead tracings.
In order to better explain the novel aspects and unique benefits of the present invention, a brief explanation of vectorcardiography and the numerous steps and processes a physician typically undergoes in order to offer a somewhat accurate diagnosis is relevant.
Vectorcardiography uses a vector description of the progress of the depolarization wave front through the heart during the P-wave or loop, the QRS wave or loop and the T-wave or loop as described and illustrated in U.S. Pat. No. 4,587,976, for example, particularly in reference to FIGS. 1 and 2 thereof. Vectorcardiography abandons the one dimension time coordinate of the ECG in favor of plots or tracings of the orientation and magnitude of the vector of the depolarization wave front on each of three planes: a vertical, frontal (X,Y) plane plotting an X-axis (right side or arm to left side or arm) against a Y-axis (head to foot); a horizontal or transverse (X,Z) plane plotting the X-axis against a Z-axis (anterior-posterior); and a vertical, sagittal (Y,Z) plane plotting the Y-axis against the Z-axis as shown in FIG. 2. The resultant xyz-vector is often characterized as comprising the mean P-wave vector, the mean QRS vector and the mean T-wave vector over a cardiac cycle. Each xyz-vector traces a loop during the time of occurrence of the P-wave, QRS complex and T-wave of FIG. 1. In simplified terms, at least three orthogonal ECG signals are simultaneously obtained from at least three orthogonal ECG leads that are generally co-planar with the frontal X,Y plane, the transverse X,Z plane, and the sagittal Y,Z plane. Signal pairs are combined to form the frontal X,Y plane vector or z-vector, the transverse or horizontal X,Z plane vector or y-vector, and the sagittal Y,Z plane vector or x-vector as shown in FIG. 2. The visual presentation and measurement of the xyz-vector in 3-D space is difficult. Consequently, the planar x-vector, y-vector and z-vector are typically simultaneously displayed employing three CRT displays or a split screen CRT display. The trained physician viewing the displays can diagnose the state of the heart from distinctive characteristics of the displayed planar vectors.
Calculations of planar x-vector, y-vector and z-vector and a resultant xyz-vector from lead systems are set forth in U.S. Pat. No. 4,569,357, for example, as referenced in U.S. Pat. No. 4,587,976. Systems for developing and displaying the xyz-vector from four or six ECG skin electrodes are disclosed in U.S. Pat. Nos. 4,478,223 and 5,458,116. A 3-D display of the xyz-vector is disclosed in U.S. Pat. No. 6,052,615.
Referring again to FIG. 2, the right and left ventricles are depolarized typically within a period of about 0.08 seconds (one normal QRS interval) and, as a result an electrical force is generated which is characterized by a QRS vector which depicts both the size and direction of the electrical force at any point in time. The normal plane for the QRS vector (i.e., the normal plane of activation) is the same as the QRS cycle, i.e., generally perpendicular to the frontal X, Y plane and slanted along the axis of the heart. It has been found that the force and direction of the QRS vector shown in FIG. 2 provides an accurate representation of how the heart is functioning over the period of the QRS interval.
FIG. 2 also depicts the smaller T-wave loop of the composite spatial xyz-vector of a normal heart. The amplitudes and spatial orientations of the T-wave vector and the QRS vector are changed in a characteristic manner in hearts having certain known cardiac disease processes.
The conception of vectorcardiography is attributed to Einthoven who determined that the QRS xyz-vector could be approximated by the z-vector projected into the frontal X, Y plane. Einthoven employed three skin electrodes specifically oriented on the body (right arm RA, left leg LL and left arm LA). The leads between these three electrodes formed a triangle known as Einthoven""s triangle. It was postulated that these ECG lead signals could always be related to a single vector in the frontal X,Y plane because a vector can be derived from any two signals added vectorally. For diagnostic purposes, these leads were later graphically translated into a triaxial system. Other leads were subsequently added to the triaxial system (termed unipolar leadsxe2x80x94aVR, aVL, and aVF) and a Hexial system was developed. For simplification purposes, the system was displayed on a circle and degrees were later assigned to the various leads of the system.
In order for a physician to determine the mean QRS vector, the physician would line up the various leads around the circle according to their positive or negative sign and magnitude and mark the transition from positive to negative on the circle. This area of transition is typically referred to as the xe2x80x9ctransitionxe2x80x9d area which when analyzing a single plane, e.g., the frontal X,Y plane, is represented by a line on the circle which separates the circle into positive and negative halves. The mean QRS vector is positioned at a right angle to the transition line on the positive side.
Using the above methodology, the direction and location of the mean QRS vector on the circle determines how the heart is functioning and allows a physician to ascertain typical heart malfunctions. For example, in a normal adult, the mean QRS vector is usually located between 0xc2x0 and 90xc2x0, i.e., between leads I and aVF on the circle. However, a left axis deviation (LAD) is characterized by the mean QRS vector being located in the 0xc2x0 to xe2x88x9290xc2x0 range and with right axis deviation (RAD) the mean QRS vector is located in the 90xc2x0 to 180xc2x0 range.
As noted in U.S. Pat. Nos. 4,136,690 and 4,478,223, it has long been known that medically significant VCGs can be produced through the use of such a three-lead system. Previous studies have already indicated merit in the VCG analysis of anomalous and ectopic beats for identifying the site of origin of ectopic beats. Such anomalous beats not only commonly result in alteration of readily apparent direction and magnitude of QRS and T-wave vectors, but also affect the direction of rotation QRS vector forces, often accompanied by abnormal delays of QRS vector inscription. The latter characteristics are not readily apparent in analog ECG signals, and thus the VCG gives additional discriminative data. The adjunctive VCG data complements the analog ECG signal data by providing a visual integrated picture of the electrical activity.
Orthogonal leads to provide these tracings were developed by Ernest Frank (see xe2x80x9cAn Accurate, Clinically Practical System For Spatial Vectorcardiographyxe2x80x9d, Circulation 13: 737, May 1956). Frank experimentally determined the image surface for one individual, and from this proposed a system using seven electrodes on the body, plus a grounding electrode. The conventional letter designations for such electrodes, and their respective positions were:
E at the front midline (anterior or ventral);
M at the back midline (posterior or dorsal);
I at the right mid-axillary line (right side);
A at the left mid-axillary line (left side);
C at a 45xc2x0 angle between the front midline and the left mid-axillary line;
CR on the neck (cranial), and
F on the left leg.
Most diagnostic vector ECG studies have been carried out using the Frank lead system or a modified McFee lead system. An alternative to the Frank lead system that required only four active electrodes (R (right arm), A, F, E), and that used a resistor network based on Frank""s image surface data was proposed in 1958 by G. E. Dower and J. A. Osborne (see xe2x80x9cA Clinical Comparison of Three VCG Lead Systems Using Resistance-Combining Networksxe2x80x9d, Am Heart J 55: 523,1958). However, the X-axis, Y-axis and Z-axis signals produced were sometimes different from those of Frank""s lead system, and the RAFE system was not adopted. Other lead systems are disclosed in the above-referenced ""116 and ""223 patents.
As described in the above-referenced ""116 and ""690 patents and illustrated in FIG. 2, the tip of the QRS vector which represents the cardiac wave front typically traces an oval or cardioid trajectory or loop during the course of each ventricular depolarization-repolarization of the cardiac cycle. Clinical studies, using data from three-lead VCG systems, have indicated the diagnostic value of the maximal QRS vector and T-wave vector which are the vectors drawn from the starting point of the loop to the farthest points of the QRS and T loops. The maximal vector should not be confused with the mean direction which is the vector equal to the sum of all of the instantaneous vectors. The absolute values of the QRS peak vector, the T-wave peak vector, and their difference are not of prime importance for diagnostic purposes, since the absolute values vary from patient to patient as well as with variations in the positioning of the electrodes on the patient. Instead, in each instance, the departures from the angles normally observed in a given patient are diagnostically significant.
The scalar representation of an abnormal supraventricular complex, particularly if nodal-originating, may appear as a bizarre waveform closely resembling a ventricular-originating arrhythmia. However, the relationship between the depolarization potentials represented by the QRS vector forces and the repolarization potential represented by the T vector forces has been proven to be nearly identical for all supraventricular originating complexes, both normal and abnormal. As a result of this fact, a first condition that can be distinguished is whether the ectopic complex is truly of supraventricular origin, the categorization of which includes the normal S-A node complexes in addition to abnormal atrial and nodal ectopic beats. Thus, it is of utmost importance and utility that the differential vector angle can initially aid in the diagnoses and categorization of supraventricular ectopics, whereas a single (scalar) lead system cannot reliably be used to do so.
Additionally, ventricular ectopic complexes of significantly different points of origin (foci) within the ventricles also display significantly different vector angles. Therefore, further categories can be set up for the purpose of identifying the relative foci of the ectopic events, and to some extent (when the lead configuration and heart position are known), the location of the foci within the heart muscle itself. Ventricular ectopi rarely originate from more than five significantly separate foci, and typically originate from one to three foci. Therefore, considerable simplification can ultimately be achieved in the overall circuit mechanization.
In the ""690 patent, two-channel, approximately orthogonal, ECG lead signals are applied to a rectangular-to-polar coordinate converter, which produces two output signals showing respectively the instantaneous magnitude and angle of the vector. Not all of the instantaneous values of the vector angles are of interest, but primarily the vector angles at the instants when the QRS and T complexes reach their peaks. These angles are then subtracted to determine the angular difference between the QRS and T vectors which henceforth are termed xe2x80x9cQRS-T anglexe2x80x9d or xe2x80x9cQRS-T vector anglexe2x80x9d.
The mean T-vector and the mean P-vector are determined in a similar manner. In fact, physicians have determined that one of the more important elements of graphically illustrating the means QRS vector and the mean T-vector is that the angle between the two vectors can be easily ascertained. This angle relates the forces of ventricular depolarization with the forces of ventricle repolarization. In a normal adult, the angle between the mean QRS vector and the mean T-vector is rarely greater than 60xc2x0 and most often below 45xc2x0.
Similarly, the mean P-vector can be determined. This enables a physician to isolate the location of the electrical direction of the excitation of the cardiac muscle of the atrium.
The above analysis has been described using a single plane, namely the frontal X,Y plane characterized by the superior, inferior, right and left boundaries of the human body. In order for a physician to analyze the overall movement of the heart muscle during depolarization and repolarization, the physician needs to analyze the vector forces along another plane, namely the transverse X,Z plane which is characterized by the posterior, anterior, right and left boundaries of the human body.
Much in the same manner as described above, six leads are positioned about the body to measure the electrical currents across the heart muscle in the transverse X,Z plane. These leads are typically called the precordial leads and are represented as VI-V6, respectively. Using the same methodology as described above with respect to the frontal X,Y plane, the location and direction of the mean QRS vector in the transverse X,Z plane can also be determined.
When the X,Y and X,Z planes are analyzed simultaneously, the mean QRS vector (and the other vectors) projects perpendicularly from the transition xe2x80x9cplanexe2x80x9d rather than the transition xe2x80x9clinexe2x80x9d of the single plane system. In other words, when the frontal plane and the horizontal plane are isolated and individually analyzed, the mean QRS transition appears as a line across the diameter of the circle. In actuality, this xe2x80x9clinexe2x80x9d is actually a xe2x80x9cplanexe2x80x9d when both systems (frontal and horizontal) are analyzed simultaneously and the mean vectors (QRS, T and P) project perpendicularly from this plane into both systems.
As can be appreciated from the above summary, the analytical process of determining the resultant QRS vector and the other vectors can be quite cumbersome and requires a physician to interpret various graphs and/or solve various formulas which tend only to frustrate the diagnostic process and which can lead to erroneous conclusions if analyzed improperly. For simplicity, most physicians analyze each system individually at first and then combine the results. However, as often is the case, the determination of the mean vectors (QRS, T-wave and P-wave) in one plane is still both time consuming and somewhat confusing. Further, trying to determine how the mean vectors project into two planes and how the angles between the vectors relate can be even more confusing.
Moreover, even if a physician can adequately analyze the various graphs and solve the various formulas to arrive at a diagnosis, 3-D representation of the location of the mean QRS vectors (and the other vectors) must be mentally visualized which requires a high degree of mental agility and can lead to misdiagnosis. Further, mentally visualizing the angles between mean vectors would be virtually impossible for even the most skilled physician. The additional problem of how these vectors change in time over the QRS interval is believed to be nearly impossible to consider by the prior methods.
Thus, although it has long formed a basis for teaching electrocardiography, vectorcardiography has never become widely used. The technique is demanding and the system of electrode placement is different from that required for the ECG. Extra work is required, and it is still be necessary to record a 12-lead ECG separately with a different placement of electrodes. However, the vector representations have been drawn for various cardiac diseases and form the bases upon which a doctor is trained to understand and explain the lead tracings from the various leads in the classic 12-Lead ECG system.
But, it is known that the VCG provides valuable diagnostic information for the initial diagnosis and follow-up of the progression of or improvement with treatment of various cardiac disease states or congenital heart defects. Numerous pathologic states may be diagnosed by means of the vectorcardiography including ischemic heart disease, dilatative cardiomyopathy, hypertrophic cardiomyopathy systolic as well as diastolic load of the ventricles, atrial dilatation and various forms of heart failure. Congenital heart defects are also characterized by specific VCG patterns. The VCG is also employed to precisely diagnose ischemic heart disease and localise the myocardial infarction. Moreover, it can be beneficial in discriminating between various types of arrhythmias, e.g., distinguishing ventricular tachycardias and malignant tachyarrhythmias from supraventricular tachycardias. Various arrhythmias and conduction disturbances such as WPW syndrome and any combination of bundle branch blocks produce specific VCG patterns.
There are many instances where it is desirable to be able to diagnose intermittent spontaneous cardiac arrhythmias in ambulatory patients. Frequently faintness, syncope, and tachyarrhythmia palpitation symptoms cannot be induced and observed by the physician in tests conducted in a clinic. For many years, such patients have been equipped with external ECG monitoring systems, e.g., the patient-worn, real time Holter monitors, that continuously sample the ECG from skin electrodes providing one or more ECG lead and record it over a certain time period. Then, the ECG data must be analyzed to locate evidence of an arrhythmia episode from which a diagnosis can be made.
As described in commonly assigned U.S. Pat. Nos. 5,312,446 and 4,947,858, the externally worn ECG recorders have inherent limitations in the memory capacity for storing sampled ECG and EGM data. Cost, size, power consumption, and the sheer volume of data over time have limited real time external Holter monitors to recording 24-hour segments or recording shorter segments associated with arrhythmias that are felt by the patient who initiates storage.
The use of the externally worn Holter monitor coupled with skin electrodes is also inconvenient and uncomfortable to the patient. The skin electrodes can work loose over time and with movement by the patient, and the loose electrodes generates electrical noise that is recorded with the EGM signal and makes its subsequent analysis difficult. It has long been desired to provide an implantable monitor or recorder that is hardly noticeable by the patient and provides the capability of recording only EGM data correlated with an arrhythmia episode that is automatically detected.
Over the last 40 years, a great many IMDs have been clinically implanted in patients to treat cardiac arrhythmias and other disorders including implantable cardioverter/defibrillators (ICDs) and pacemakers having single or dual chamber pacing capabilities, cardiomyostimulators, ischemia treatment devices, and drug delivery devices. Recently developed implantable pacemakers and ICDs have been provided with sophisticated arrhythmia detection and discrimination systems based on heart rate, the morphology and other characteristics of the atrial and ventricular EGM and other characteristics of the EGM. Most of these IMDs employ electrical leads bearing bipolar electrode pairs located adjacent to or in a heart chamber for sensing a near field EGM or having one of the electrodes located on the IMD housing for sensing a far field, unipolar EGM. In either case, the near field or far field EGM signals across the electrode pairs are filtered and amplified in sense amplifiers coupled thereto and then processed for recording the sampled EGM or for deriving sense event signals from the EGM.
In current IMDs providing a therapy for treating a cardiac arrhythmia, the sense event signals and certain aspects of the sampled EGM waveform are utilized to automatically detect a cardiac arrhythmia and to control the delivery of an appropriate therapy in accordance with detection and therapy delivery operating algorithms. In such cardiac IMDs providing pacing or cardioversion/defibrillation therapies, both analog and digital signal processing of the EGM is continuously carried out to sense the P-wave and/or R-wave events and to determine when a cardiac arrhythmia episode occurs.
For example, a digital signal-processing algorithm is employed to distinguish various atrial and ventricular tachyarrhythmias from one another. When a tachyarrhythmia episode is detected, at least selected EGM signal segments and sense event histogram data or the like are stored on a FIFO basis in internal RAM for telemetry out to an external programmer at a later time. Many of these IMDs are also capable of being operated to sample the near-field EGM across bipolar electrode pairs and the far-field EGM between a lead borne electrode and an IMD housing or can electrode. The IMD can be commanded to transmit real time EGM data of indefinite length via uplink telemetry transmissions to the external programmer when a real time telemetry session is initiated by the medical care provider using the programmer.
Implantable cardiac monitors have also been developed and clinically implanted that employ the capability of recording cardiac EGM data for subsequent interrogation and uplink telemetry transmission to an external programmer for analysis by a physician. The recorded data is periodically telemetry transmitted out to a programmer operated by the medical care provider in an uplink telemetry transmission during a telemetry session initiated by a downlink telemetry transmission and receipt of an interrogation command.
The MEDTRONIC(copyright) Reveal(trademark) insertable loop recorder is a form of implantable monitor that is intended to be implanted subcutaneously and has a pair of sense electrodes spaced apart on the device housing that are used to pick up the cardiac far field EGM which in this case is also characterized as a xe2x80x9csubcutaneous ECGxe2x80x9d. The Reveal(trademark) insertable loop recorder samples and records one or more segment (depending on the programmed operating mode) of such far field EGM or subcutaneous ECG signals when the patient feels the effects of an arrhythmic episode and activates the recording function by applying a magnet over the site of implantation. For example, the storage of a programmable length segment of the EGM can be initiated when the patient feels faint due to a bradycardia or tachycardia or feels the palpitations that accompany certain tachycardias.
The most recently stored segment or segments of episode data is transmitted via an uplink telemetry transmission to an external programmer when a memory interrogation telemetry session is initiated by the physician or medical care provider using the programmer. Aspects of the RevealTm insertable loop recorder are disclosed in commonly assigned PCT publication WO98/02209.
More complex implantable monitors and pacemaker implantable pulse generators (IPGs) of this type but having more electrodes arranged in a planar array on the device housing are disclosed in commonly assigned U.S. Pat. Nos. 5,331,966, 6,115,628, and 6,230,059. Three or more electrodes are employed to provide a pair of orthogonal sensed EGM or xe2x80x9csubcutaneous ECGxe2x80x9d signals at the subcutaneous implantation site. A lead can be employed in a disclosed pacemaker embodiment to locate a bipolar electrode pair in a heart chamber to provide an additional near field EGM sense signal from which the P-wave or R-wave can be sensed (depending on the location of the bipolar electrode pair) and through which pacing pulses can be applied to the atrium or ventricle.
Recording of the near field and far field EGM episode data can be invoked automatically by detection of a bradycardia or satisfaction of tachyarrhythmia detection criteria or can be manually commenced by the patient using an external limited function programmer or can be commenced by the physician using a full function programmer.
Various types of cardiac EGM data are collected in further implantable cardiac monitors or other IMDs having monitoring capabilities including those disclosed in U.S. Pat. Nos. 5,404,877, 5,425,373, 5,497,780, 5,556.419, 5,740,811 and 5,810,739.
However, it does not appear that IMD systems have been utilized to develop VCG information for diagnostic reasons or to distinguish tachyarrhythmias from normal, high rate, sinus rhythms or to detect occurrence or degree of myocardial infarction.
While apparently generally acceptable for their intended purposes, so far as is known, none of the prior art IMDs collects EGM data from which a VCG signal in 3-D xyz-vector format or 2-D projections in the sagittal, frontal and/or horizontal planes that can be stored, displayed or employed for diagnostic purposes.
Accordingly, the present invention provides an IMD with the capacity of deriving VCGs signifying the progress of the depolarization and repolarization wave front signal through the heart during the PQRST segment of the heart cycle, storing such VCGs in memory, and/or employing characteristics of the VCGs in the determination of the state of health of the heart.
The present invention provides for the derivation of vector magnitude and orientation data (as polar coordinates, for example), of high rate PQRST electrogram segments of heart cycles. The polar coordinate data can be mathematically plotted over the time of occurrence of the sensed PQRST electrogram as at least one of an x-axis vector projected into the reference sagittal plane as a sagittal VCG, a y-axis vector projected into the reference horizontal plane as a horizontal VCG, a z-axis vector projected into the reference frontal plane as a frontal VCG, and an xyz-vector in 3-D space. The loops plotted by each of the vectors can also be derived.
In accordance with a further aspect of the present invention, a gain factor that compensates for the angular deviation of the internal lead vector out of coplanar relation with the at least one of the reference sagittal, horizontal, and frontal planes of the body that the at least one of the sagittal vectorcardiogram, horizontal vectorcardiogram, and frontal vectorcardiogram is calculated and employed to correct the PQRST electrogram.
The novel elements believed to be characteristic of the present invention are set forth in the appended claims. The invention itself, together with additional objects and attendant advantages, will best be understood by reference to the following detailed description, which, when taken in conjunction with the accompanying drawings, describes presently preferred embodiments.